1. Field of the Invention
The present invention relates to light emitting diode arrays which can be used to produce gray scale light images which expose a photosensitive medium.
2. Description of the Prior Art
The term "monolithic device" as used herein refers to a device that is formed on a single semiconductor crystal chip. The semiconductor chip can be formed from, for example, of extrinsic gallium arsenide.
The term "light emitting diode" (LED) as used herein refers to a semiconductor device that emits radiation in the optical spectrum (i.e., infrared through ultra violet) in response to an applied forward bias voltage from an external power source. An LED has a p-n junction provided by two extrinsic semiconductors. When forward biased by an external power source, such a diode emits optical radiation. LEDs are attractive sources of optical radiation because they are easy to form, inexpensive to make, highly efficient, and reliable. They are widely used as miniaturized lamps and in display devices such as calculators and watches.
High-density, linear LED arrays can be fabricated in a monolithic device and used as light sources in electronic printing applications. In such applications, light from the diodes of an array forms a line of light images at corresponding pixels of an image zone. A photosensitive medium is positioned (a line at a time) at the image zone to be exposed by these light images. With existing technology, it is possible to fabricate an array of light emitting diodes on a single gallium arsenide chip. Discrete regions of the array constitute a picture element portion (pixel) of the array. Each array pixel includes a single LED which can illuminate a particular image zone pixel. Two-level exposures of image zone pixels are produced by selectively turning array LEDs on and off.
Although this arrangement works well for black and white light images such as solid area and alpha-numeric images, at least three different exposure levels (levels of gray scale light images) per image zone pixel are required for good-quality, continuous-tone images to be formed on a photosensitive medium. One way this is accomplished is by having two separate linear LED arrays. Each array would have a single diode assigned to an image zone pixel. The LEDs of one array are used to produce the brightest light images (for D.sub.max exposure) at corresponding image zone pixels; the LEDs of the other array are used to illuminate a halftone screen to produce lower brightness level light images (for intermediate half-tone exposure) at corresponding image zone pizels. If each array has n (a real positive integer) LEDs, a total of 2n LED are needed. Also, 2(n+1) connections are required for the two arrays if each of them has a common-cathode. The light from the two arrays must be optically aligned so that corresponding LEDs illuminate the same image zone pixel.